The present invention relates to an electrophotographic toner composition, and more particularly, to an electrophotographic toner composition providing lower gamma.
Numerous considerations exist in designing a toner composition useful in electrophotography. Toners are usually made of resins or resin blends in which a pigment such as carbon black is dispersed. The toner is designed to accept a charge of the correct polarity. In dry toners, this occurs when the toner is brought into rubbing contact with a carrier material, a ferromagnetic fiber of a magnetic brush, or the like. In liquid toners, the charge is acquired from the suspending liquid. In the reproduction of high contrast copies such as letters by cascade development, it is desirable to select toner particles and carrier materials so that their mutual electrification is governed in most cases by the distance between their relative positions in the triboelectric series. When otherwise compatible toner particles and carrier materials are removed from each other in the triboelectric series by too great a distance, the resulting images are very faint because the attractive forces between the carrier and toner particles compete with the attractive forces between the electrostatic latent image and the toner particles.
Most office copiers operate at a relatively high gamma to improve the reproduction of light pencil lines and other low contrast images which are of importance in this environment Gamma is conventionally defined as the slope of the linear part of a tonal reproduction curve wherein input density is plotted on the abscissa and the output density is plotted on the ordinate. The saturation density occurs beyond the linear region where an increase in input density produces no increase in output density. Electrophotographic toner compositions which provide relatively high gamma are unsuitable for use in several processes including reproduction of continuous tone images, e.g., photographs, and, in the presence of machine jitter, half tones. Thus, the need exists for an electrophotographic toner composition providing lower gamma and meeting the aforementioned considerations.